Let's make sure to compare these mics properly before suspecting something may be wrong:

Do you have the original U87 (pre-1986) or the newer U87Ai? If these are pre-"A" models, what transformer output strapping is set? Are the pads engaged?

If these are "A" models, their output should be roughly equal to a U67 strapped for 200 Ohms.If they are pre- "A" a 50Ω strapping and/or pads engaged can easily explain the output difference.

Let's start there and then we talk.

But one thing is for sure: your U67 did not secretly go to the refrigerator late at night, gaining 6dB! Mics never increase output over time. If they would, we would have a new form of cheap energy production.

Thanks Klaus. Indeed. These are 87 (non A) versions. They are both strapped for 200ohm output impedance. But I neglected to check the pad resistors. I just looked now, and the resistors (R20 and R21) are not bridged. So that explains my 6db, correct?

Assuming the transformers in both models are wired for 200Ω output impedance, then the nominal output levels for a SPL of 1 Pa (94 dBspl) are 8 mV for the U 87 and 20 mV for the U 67 (cardioid), 11 mV (omni) and 14 mV (∞ or figure eight). With other words, the U 67 will have 3 dB higher output level in omni, 8 dB more in cardioid, and 5 dB more in figure eight than the U 87!